Peroxide value measures the hydroperoxides formed when oxygen attacks the unsaturated fatty acids in an oil. It is determined by ISO 3960 and reported in milliequivalents of active oxygen per kilogram of oil. It is the standard first indicator of oxidative deterioration in a seed oil, and it is routinely misread.
Why it works this way
Oxidation runs in two stages. Hydroperoxides form first, which is what the test detects, and then break down into secondary products, the aldehydes and ketones that produce the rancid smell. Because the second stage consumes what the first stage produced, the peroxide value rises, peaks and then falls. A low figure therefore means either a fresh oil or a badly degraded one, and the number alone cannot distinguish them.
Two other properties make it fragile as a contractual parameter. It is sensitive to how the sample was drawn, stored and transported before testing, so a delayed sample gives a higher result than the oil deserves. And it describes a moment rather than a trajectory: an oil with a low peroxide value and no remaining antioxidant capacity will fail faster than one with a slightly higher value and intact tocopherols.
The companion parameter is acid value, or free fatty acid content, determined by ISO 660. It measures hydrolytic rather than oxidative breakdown and largely reports on seed quality and on the interval before pressing. Free acidity and peroxide value together describe an oil far better than either alone.
What follows in practice
Specify peroxide value with the method, the maximum at delivery rather than only at production, and the production date on the certificate. Where oxidative stability drives shelf life, add an accelerated stability test such as a Rancimat induction time. Cold-pressed seed oils sets out the full parameter set, and how to store cold-pressed oils covers what happens after delivery.